Method of densifying wood



Jan. 19, 1954 c. c. HERITAGE 2,665,453

' METHOD OF DENSIFYING wooo Filed Feb. 21, 1949 PLATEN HEATING CIRCUIT LIGHT PRESSURE HIGH FREQUENCY HEATING cmcun THERMO- COUPLE CIRCUIT L PLATEN HEATING CIRCUIT HEAVY PRESSURE HIGH FREQUENCY HEATING CIRCUIT DE GUARDS En I TU A/D Patentecl Jan. 19, 19 54 I af'midv i not lower than the extent desired, and the sample should be subjected to the compression treatment of step II as quickly thereafter as possible and while the wood is still substantially at the maximum temperature developed in the moisture reducing treatment. In other words, the degradation of the lignin may be considered theoretically to be advanced in accordance with the product of (1) the time of exposure at, or association with, a moisture content reduced below that of the green wood, and (2) the extent of reduction of the moisture content below its normal green wood value. Since the moisture must necessarily be reduced to the range specified for the compression treatment, the only opportunity of capturing the native reactivity of the lignin with a minimum of degradation is to accomplish the moisture reduction process as quickly as possible and to no greater extent than n necessary, and then to apply the pressure of the compression treatment in quick sequence. A third factor which undoubtedly influences the degradation of the native lignin is the subjection thereof to heat, particularly at reduced moisture content. It is accordingly desirable to avoid all excess heat in the moisture reduction step and to minimize the time of exposure to heat. However, since it is desirable, and facilitates the compression step, to apply the pressure under heat, it is accordingly advantageous to utilize all the heat absorbed by the wood in the moisture reducing step in order further to conserve time and reduce the total amount of heat absorbed.

In the second step of the process, which is conducted as quickly as possible after step I, as explained above, the wood is compressed under the influence of sufficient heat and pressure until its volume has been reduced to a fraction of its original volume, and then cooled while maintaining pressure. This step has for its objective the obyious result of increasing the density, and also to utilize the lignin, in the presence of the appropriate moisture content, as a combined plas- 'ticizer and resin to permanently bind the cellular tissues of the compressed fibrous materialin its compressed condition to the maximum extent possible. It is believed that the lignin, or a wood substance generally regarded as lignin or as associated with the lignin, has thermoplastic properties and that the thermoplasticity is much improved by the presence of water. Whether for mechanical or chemical reasons, the optimum range of moisture content has been found by experience to be from about to the fiber saturation point.

f It is further believed that the lignin or a wood substance generally regarded and considered as lignin, also functions during this process step, somewhat in the manner of the condensation product of a thermosetting resin so that it first flows, and then cures, as the polymerization advances under the action of heat and the passing of time. The greater the extent to which this theoretical action of the lignin is accomplished,

.- the more eifective will be the resistance of the compressed wood to subsequent swelling or it expeditious to carry out the process in two distinct phases using a different apparatus for each step. The first of these phases of the process is identical with step I described above, and the second phase is identical with step II described above.

The process is typically illustrated in the treatment of a one-inch thick board of either flat or edge grain green wood having a moisture content far in excess of the fiber saturation value. The first phase of the process comprises the rapid heating of this green board in a, high frequency dielectric field between metallic condenser plates to a uniform temperature slightly above 212 F. in order to reduce the moisture content to a value between 15% and the fiber saturation point in the shortest possible time. The moisture content at fiber saturation, of course, varies with the specific gravity of the wood, and the above value of 30% therefore represents the commonly accepted average value. The rate of heating is adjusted in accordance with the rate of moisture loss from the wood to produce the desired moisture content at the end of the first phase by the time the temperature reaches approximately 220" FL, by controlling the electrical power output from a high frequency oscillator which is absorbed by the wood as a dielectric body. The driving oif of the moisture is accompanied by formation of a cloud of steam, with the size of the cloud diminishing as the moisture content is reduced below the fiber saturation point.

The moisture content of the wood during the practice of step I may be conveniently determined by the variation in voltage across the condenser plate electrodes on opposite sides of the wood. Inasmuch as the power factor and capacitance decrease as the wood loses its moisture, the voltage across the plates for constant power input increases as i the wood dries. Thus, by keeping the power input constant, the voltage readings may provide a direct indication of the moisture content. The fiber saturation point is very easily determined in this manner because of the electrical conductivity characteristics of wood containing free water in its grosser capillary structure. The electrical conductivity characteristics of Wood do not vary as much with varying amounts of moisture content above the fiber saturation point, as they do in response to varying amounts of moisture content below the fiber saturation point. As completely or partially water filled wood is dried, the electrical conductivity changes ratherslowly until the fiber saturation point is reached, the conductivity change for the entire range from completely water filled condition of the wood to fiber saturation point being only about 20 fold. As the wood dries from fiber saturation point, at about.30% moisture content to 7% moisture content, there is an approximately linear relationship between the moisture content and the logarithm of the electrical conductivity which produces a conductivity change over this range of over 100,000 fold. The break in the conductivity-moisture content relationship is thus so sharp that such variables as species, specific gravity of wood and variations in ash content, all have but slight effect on the conductivity in comparison with the effect of the moisture content. The rapid change in conductivity from 30% down to 7% moisture content also makes it relatively easy to determine the moisture content at any point in the range between 30% and 15% with which the present incility for quick heating of the platens by steam or electrical resistance to avoid loss of heat from the wood between the two steps of the process. In this procedure it is not necessary to handle the wood in hot condition; there is a minimum elapse of time between the completion of step I and the beginning of step II; and there is no potential across the plates to damage the wood while it is under high pressures.

Figures 1 and 2 illustrate how the two steps of the process may be carried out expediently in a single apparatus such as an electrically heated hot press, as described hereinabove. In Figure '1 light pressure is applied to the unheated platens and the wood is dried rapidly to the desired moisture content by the action of the high frequency heating circuit. Figure 2 illustrates the second step wherein heavy pressure is applied while the platens are heated by resistance elements in the platen heating circuit. The high frequency circuit is deenergized at this time.

Douglas fir boards compressed in the manner described above develop unexpected strength properties. As has been hereinabove mentioned, the strength properties of wood in general have heretofore been considered to be roughly proportional to the density of the wood regardless of the kind of wood. This principle has also been extended to apply to compressed wood treated according to prior art processes. However, using the process of the present invention, a green, flatgrained, Douglas fir board one inch thick having an original specific gravity of when cornpressed to double its specific gravity, develops on the average a modulus of rupture in bending and a toughness by impact test of nearly two and onehalf times the corresponding values of the orig inal wood, and a compressive strength in a direction radial to the grain 16 times that of the orlginal wood. The percent of swelling, as defined by change in thickness with respect to the original compressed thickness, for a number of samples, amounted to only 3 to 12% after 24 hours submersion in water. On the other hand, samples of Douglas fir boards compressed without regard to the critical conditions of the present process, to a specific gravity twice that of the original wood quickly swell to approximately the original thickness (90 to 100%) when immersed in water.

Douglas fir treated in accordance with the process of the invention is somewhat darkened, and its natural beauty is enhanced. When the compressed wood is sawed across the grain, the sawed end exhibits a fine, natural dark polish. Microscopic examination of a section perpendicular to the grain reveals that the spring wood is almost completely collapsed, the cells of the summer wood are partially collapsed, and the rays telescoped accordion fashion. The tendency to check and split is substantially reduced, and its wearing qualities, particularly for use as flooring material or the like, are greatly improved as indicated by the above comparisons with natural wood.

Western red cedar is more difiicult to treat by the present process and its response to the treatment is not as uniform as that of Douglas fir, but does respond satisfactorily to the treatment with remarkable results. Cedar is li hter in weight and softer than Douglas fir and hence compresses more under the same conditions of pressure and temperature. For instance, under pressure and temperature conditions effective to reduce Douglas fir to about .4 of its original thickness'the cedar is compressed to .2 of its original thickness. Good results have been produced with cedar compressed from an original specific gravity of .35 to a compressed specific gravity of 1.21, which is somewhat in excess of a 3.5 fold increase in density after allowing for the loss of moisture. Specimens have thus been produced having as low as 1% swelling defined by change in thickness after 24'hours submersion with respect to the original compressed thickness. The most remarkable improvement, however, is shown in the compressive strength in a radial direction with respect to the grain Where strengths have been developed 45 times that of the original Wood.

Since the specific gravity of wood substance itself is about 1.46, the exceptionally high strength properties developed by the present treatment cannot be accounted for merely on the basis of proportionality with respect to specific gravity according to the heretofore generally accepted rule. On this basis the strength properties of Douglas fir (original specific gravity .59) could not be increased beyond an ultimate limit of about two and one-half times its original strength and Western red cedar could not be improved beyond four or five times its original strength, even if the wood were compressed to a solid condition with all the capillary spaces completely collapsed. It is, therefore, believed that the present process not only compresses the wood permanently to a denser condition with less energy than prior art processes, but also effects a marked change inthe nature of the Wood substance itself by the utilization of the plasticizing and the binding or resinous properties of the lignin or other reactive wood substances in the form nearest its original constitution or composition of its native state where its reactivity has been least altered, and it is this discovery that accounts for the greatly improved strength properties imparted to the wood, particularly in regard to its compressive strength.

The present process is particularly advantageous to increase the utilization of small pieces of wood which might otherwise be discarded as waste material or converted into material of less intrinsic value. Although wood thus treated is exceedingly hard and tough it may nevertheless be sawed and worked with the usual woodworking tools. When the surfaces are sanded they display a dark, almost lustrous finish, as though they had been stained and shellacked, making the wood particularly attractive as well as durable.

It is to be understood that the invention is not limited to the specific apparatus mentioned herein for carrying out the process, for various heating and pressing equipments may be devised by those skilled in the art to accomplish the general objectives stated in the specification. Neither'is it intended that the invention be limited by the explanations of theory expressed herein should subsequent investigation show them to require alteration' It is to be further understood that various modifications of the process may be necessary in order to apply best the broad principles of the invention to different types of Wood of difierent thicknesses and surface areas.

What is claimed is;

1. The method of treating green wood comprising rapidly heating the wood to reduce the moisture content to a predetermined amount in a range between 15% and the fiber saturation point and for plasticizing the native lignin in the presence of the remaining moisture, and immediately thereafter, before loss of a substantial 9 amount of the heat applied to the wood for reducing the moisture content, applying pressure to compress the wood to a predetermined thickness dimension.

2. The method of improving the structural properties of wood comprising rapidly heating green wood to reduce the moisture content thereof to a predetermined value within the range from to the fiber saturation point for plasticizing the native thermoplastic and thermosetting substances in the wood in the quickest possible time in order to reduce to a minimum the degrading effects on said native thermoplastic and thermosetting substances of heat and time and existence in association with less than the moisture content of green wood, and immediately thereafter, with a minimum lapse of time and drop in temperature of the wood, applying pressure to the wood in hot condition to compress the same to a reduced volume, cooling the wood while maintaining the pressure to produce stable, permanent densification of the wood.

3. The method of improving the structural 'properties of wood comprising rapidly heating green wood to reduce the moisture content thereof to a predetermined value within the range from 15% to the fiber saturation point for plasticizing the native thermoplastic and thermosetting substances in the wood in the quickest possible time in order to reduce to a minimum the degrading efiects on said native thermoplastic and thermosetting substances of heat and time and existence in association with less than the moisture content of green wood, and immediately thereafter, with a minimumlapse of time and drop in temperature of the wood, applying pressure to collapse and partially collapse the wood cells and to flow the native thermoplastic and thermosetting substances in the wood under the influence of the remaining original moisture content, cooling said wood to substantially room temperature while maintaining the pressure to set said substances and hold the wood cells permanently in said collapsed and partially collapsed condition and then removing said pressure.

4. The method of permanent densification of wood involving the utilization of a part of its original moisture content as a plasticizer for thermoplastic and thermosetting substances in the wood, comprising rapidly heating green wood uniformly throughout its mass to a temperature sufi'iciently high to plasticize native thermoplastic and thermosetting substances in the wood and to reduce the moisture content of the wood to a moisture content ranging from about 15% to the fiber saturation point, then immediately subjecting the heated wood to a pressure and temperature sufiicient to compress the wood and flow said substances, and thereafter cooling the wood under pressure.

5. The method of improving the structural characteristics of wood comprising rapidly heating green wood to approximately220 F. while retaining from 15% to 30% of the original moisture content of the wood, immediately subjecting said wood to a pressure of from 800 to 2000 pounds per square inch at a temperature of from 300 F. to 375 F., then reducing the wood to room temperature, and then removing said pressure.

6. The method of permanent densification of wood comprising rapidlyheating green wood containing its original moisture in an alternating electrostatic field to a temperature slightly in excess of 212 F. and thereby reducing the moisture content to a value within the range of from 15% to fiber saturation, immediately subjecting the wood to pressure for a time sufficient to flow the native thermoplastic and therosetting substances in the wood, and then cooling the wood under pressure.

7. The method of permanent densification of wood comprising rapidly heating green wood containing its original moisture in an alternating electrostatic field to a temperature of approximately 220 F. and reducing the moisture content to a value in the range between 15% to 30%, immediately heating the wood to a temperature in the range between 300 F. and 375 F. and compressing the wood at a pressure in the range between 800 and 2000 pounds per square inch, and then cooling the wood under pressure.

8. The method of permanent densification of wood comprising rapidly heating green wood containing its original moisture in an alternating electrostatic field to a temperature slightly above 212 F. while reducing the moisture content to a value in the range between 15% and 30% at atmospheric pressure, then heating the wood to a temperature in the range between 300 F. and 375 F. and compressing the wood at pressures of from 800 pounds to 2000 pounds per square inch for approximately 14 to 20 minutes, then cooling the wood under pressure to approximately room temperature, and thereafter reducing the pressure to atmosphere in not less than five minutes.

CLARK C. HERITAGE.

References Cited in the fileof this patent UNITED STATES PATENTS Number Name Date 77,777 Spaulding May 12, 1868 1,952,664 Esselen Mar. 27, 1934 2,089,644 Samsonow Aug. 10, 1937 2,108,920 Humiston Feb. 22, 1938 2,231,457 Stephen Feb. 11, 1941 2,304,958 Rouy Dec. 15, 1942 2,412,523 Lundstrom Dec. 10, 1946 2,529,862 Bilhuber Nov. 14, 1950 2,543,618 Wood Feb. 27, 1951 FOREIGN PATENTS Number Country Date 5,973 Great Britain Mar. 17, 1896 

1. THE METHOD OF TREATING GREEN WOOOD COMPRISING RAPIDLY HEATING THE WOOD TO REDUCE THE MOISTURE CONTENT TO A PREDETERMINED AMOUNT IN A RANGE BETWEEN 15% AND THE FIBER SATURATION POINT AND FOR PLASTICIZING THE NATIVE LIGNIN IN THE PRESENCE OF THE REMAINING MOISTURE, AND IMMEDIATELY THEREAFTER, BEFORE LOSS OF A SUBSTANTIAL AMOUNT OF THE HEAT APPLIED TO THE WOOD FOR REDUCING THE MOISTURE CONTENT, APPLYING PRESSURE TO COMPRESS THE WOOD TO A PREDETERMINED THICKNESS DIMENSION. 